As the cable television (CATV) infrastructure evolves, the upper boundary of the return band frequency in a CATV system may be extended by increasing the frequency of the split between the return band (e.g., upstream channel) and the forward band (e.g., downstream channel); this allocates more bandwidth to the upstream channel. The return band is dedicated to the transport of signals from customer premise equipment (e.g., set-top boxes or other components) to the CATV head-end, and the forward band is dedicated to the transport of signals from the CATV head-end to the customer premise equipment. Extending the return band will enable more upstream bandwidth capacity due to the increased upstream spectrum. However, reallocating spectrum to the upstream consumes a portion of the spectrum previously dedicated to the downstream path. Thus, increasing the upper boundary of the return band frequency may interfere with CATV components that require downstream communication signals that fall within the frequency reallocated to the return band. One subset of these downstream communication signals are generally referred to as out of band signaling. In this case, many legacy components dependent on downstream communication signals in the reallocated band would become inoperative and could require replacement, resulting in a large expense for system operators.
In an example DOCSIS implementation described for purposes of illustration, consider implementations in which the return band is extended from 42 MHz or 65 MHz to approximately 200 MHz (called high-split), or higher. There may be a significant population of devices, such as legacy set top boxes, that require forward path out of band (OOB) communication signals in the forward path below 200 MHz, which is a portion of the spectrum that is now reallocated to the return band. The OOB communication signals may be control signals or other information that the head-end sends to the set top boxes. Thus, increasing the upper boundary of the return band frequency to 200 MHz may interfere with the legacy set top boxes that receive downstream communication signals below 200 MHz. Typically, these set top boxes cannot be re-tuned to receive out of band signals at the higher frequencies required. For example, the receivers for the out of band signals for a legacy set top box are normally tuned to 70-130 MHz, and typically the legacy downstream out of band STB control channel is located at 75 MHz. Thus, if the upstream frequency band on the upstream cable plant is extended, the legacy set top box out of band receivers will no longer receive their required signals. That is, the legacy downstream out of band control channel is cut off by the high-split upstream frequency band. The continuing need for out of band communications below 200 MHz represents a barrier to expanding DOCSIS bandwidth and may deter further use of a DOCSIS compliant system. Replacing all legacy set top boxes prior to a migration to an advanced DOCSIS system may be logistically (and financially) undesirable.